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• Axis-Aligned Bounding Boxes vs Bounding Ellipse

  - by Griffin

  Why is it that most, if not all collision detection algorithms today require each body to have an AABB for the use in the broad phase only? It seems to me like simply placing a circle at the body's centroid, and extending the radius to where the circle encompasses the entire body would be optimal. This would not need to be updated after the body rotates and broad overlap-calculation would be faster to. Correct? Bonus: Would a bounding ellipse be practical for broad phase calculations also, since it would better represent long, skinny shapes? Or would it require extensive calculations, defeating the purpose of broad-phase?

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• Finding the contact point with SAT

  - by Kai

  The Separating Axis Theorem (SAT) makes it simple to determine the Minimum Translation Vector, i.e., the shortest vector that can separate two colliding objects. However, what I need is the vector that separates the objects along the vector that the penetrating object is moving (i.e. the contact point). I drew a picture to help clarify. There is one box, moving from the before to the after position. In its after position, it intersects the grey polygon. SAT can easily return the MTV, which is the red vector. I am looking to calculate the blue vector. My current solution performs a binary search between the before and after positions until the length of the blue vector is known to a certain threshold. It works but it's a very expensive calculation since the collision between shapes needs to be recalculated every loop. Is there a simpler and/or more efficient way to find the contact point vector?

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• Simulating an object floating on water

  - by Aaron M

  I'm working on a top down fishing game. I want to implement some physics and collision detection regarding the boat moving around the lake. I would like for be able to implement thrust from either the main motor or trolling motor, the effect of wind on the object, and the drag of the water on the object. I've been looking at the farseer physics engine, but not having any experience using a physics engine, I am not quite sure that farseer is suitable for this type of thing(Most of the demos seem to be the application of gravity to a vertical top/down type model). Would the farseer engine be suitable? or would a different engine be more suitable?

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• How would I be able to get a game over screen using the pause function?

  - by Joachim Velzel

  I am having problems with my snake game, when the snake collides with itself it draws a "game over" image in the background, but only while it's colliding with itself. I want it to behave like the pause function, so that as soon as the snake collides with itself it draws an image on the screen and stops the game play. And then how would you be able to restart or to quit the game? I just have this for the detection at the moment: if (snakeHeadRectangle.Intersects(snakeBodyRectangleArray[bodyNumber])) { spriteBatch.Draw(textureGameOver, gameOverPosition, Color.White); } Thanks

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• Dynamic obstacles avoidance in navigation mesh system

  - by Variable

  I've built my path finding system with unreal engine, somehow the path finding part works just fine while i can't find a proper way to solve dynamic obstacles avoidance problem. My characters are walking allover the map and collide with each other while they moving. I try to steering them when collision occurs, but this doesn't work well. For example, two characters block on the road while the third one's path is right in the middle of them and he'll get stuck. Can someone tell me the most popular way of doing dynamic avoidance? Thanks a lot.

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• Boat passing under a bridge in a 2D tile based RTS

  - by aleguna

  I'm writing a 2D tile based RTS. And I want to add a 'pseudo 3D' feature to it - bridges over the rivers. I havent't start any coding yet, just trying to think how it fits the collision detection model. A boat passing under the bridge and a unit moving over the bridge will eventually occupy the same cell on the map. How to prement them from colliding? Is there a common approach to solve such a problem? Or I need to implement a 3D world to do this?

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• Keyboard-shortcut key-press-detection sensitivity settings

  - by Juve

  last week I switched from Ubuntu 10.10 to 12.04 and after setting up my keyboard shortcuts, e.g., CTRL+ALT+E for my favorite editor and CTRL+ALT+X for the terminal, I noticed that the behavior when pressing the appropriate keys changed somehow. I know this is very subjective and I am not 100% percent sure if I am suddenly just too lazy when using my keyboard, but here is what I noticed: To run your shortcuts, you usually press the modifiers first and in addition press the alphanum key. Now, if I hold the modifiers down very consciously and press the alphanum key afterwards everything works fine. However, I noticed that I may often release the modifiers a bit too early. In Ubuntu 10.10 (metacity/compiz) my keyboard shortcuts would still execute and my tools would pop up. This does not work anymore in 12.04. Nevertheless, I still believe the old behavior to be more intuitive and would like to have it back. I a nutshell: Is there a parameter to control the shortcut-key-press detection behavior? I already searched the ubuntu keyboard options and searched for "keyboard" in gconf-editor but could not find any hints so far.

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• Ignore collisions with some objects in certain contexts

  - by Paul Manta

  I'm making a racing game with cars in Unity. The car has a boost/nitro powerup. While boosting, I wouldn't want to be deviated when colliding with zombies, but I do want to be deviated when colliding with walls. On the other hand, I don't want to ignore collision with zombies, because I still want to hit them on impact. How should I handle this? Basically, what I want is for the car to not rotate when colliding with certain objects.

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• From where does the game engines add location of an object?

  - by Player

  I have started making my first game( a pong game )with ruby (Gosu). I'm trying to detect the collision of two images using their location by comparing the location of the object (a ball) to another one(a player). For example: if (@player.x - @ball.x).abs <=184 && (@player.y - @ball.y).abs <= 40 @ball.vx = [email protected] @ball.vy = [email protected] But my problem is that with these numbers, the ball collides near the player sometimes, even though the dimensions of the player are correct. So my question is from where does the x values start to count? Is it from the center of gravity of the image or from the beginning of the image? (i.e When you add the image on a specific x,y,z what are these values compared to the image?

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• How are trajectories calculated and transmitted to other players in Multi-Player ?

  - by giulio

  I play alot of COD4. And can see tracers for gunfire, missles, care packages fall from helicopters etc. There is alot of activity. I am curious to know the algorithm (at a high level) that manages all this action when you have 20 people on a map shooting each other to death ? This question touches on the subject but doesn't ask for a more in-depth answer as to how you the developers go about calculating and transmitting movement and collision detection for projectiles, be it missles/bullets or any other object that is flying through the air in real-time.

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• What has the most efficient intersection test against an AABB tree - OBB, Cylinder or Capsule?

  - by identitycrisisuk

  I'm currently trying to find collisions in 3D between a tighter volume than an AABB and a tree of AABB volumes. I just need to know whether they are intersecting, no closest distance or collision response. An OBB, Cylinder or Capsule would all roughly fit these purposes but Cylinder and Capsule were the first thing I thought of, which I have found little information about detecting intersections online. Am I right in thinking that they would always be more complex to perform Separating Axis Tests on even though they might seem like simpler shapes? I figure by the time I get my head around SAT for curved shapes I could have done the thing with OBBs but I wanted to find out for sure.

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• OpenGL Get Rotated X and Y of quad

  - by matejkramny

  I am developing a game in 2D using LWJGL library. So far I have a rotating box. I have done basic Rectangle collision, but it doesn't work for rotated rectangles. Does OpenGL have a function that returns the vertices of rotated rectangle? Or is there another way of doing this using trigonometry? I had researched how to do this and everything I found was using some matrix that I don't understand so I am asking if there is another way of doing this. For clarification, I am trying to find out the true (rotated) X,Y of each point of the rectangle. Let's say, the first point of a rectangle (top,left) has x=10 y=10.. Width and height is 100 pixels. When I rotate the rectangle using glRotatef() the x and y stay the same. The rotation is happening inside OpenGL. I need to extract the x,y of the rectangle so I can detect collisions properly.

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• How to avoid circular dependencies between Player and World?

  - by futlib

  I'm working on a 2D game where you can move up, down, left and right. I have essentially two game logic objects: Player: Has a position relative to the world World: Draws the map and the player So far, World depends on Player (i.e. has a reference to it), needing its position to figure out where to draw the player character, and which portion of the map to draw. Now I want to add collision detection to make it impossible for the player to move through walls. The simplest way I can think of is to have the Player ask the World if the intended movement is possible. But that would introduce a circular dependency between Player and World (i.e. each holds a reference to the other), which seems worth avoiding. The only way I came up with is to have the World move the Player, but I find that somewhat unintuitive. What is my best option? Or is avoiding a circular dependency not worth it?

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• Adding a small slide when player releases left/right key

  - by Dave

  the aim is for the player object to slow down and stop instead of just stopping dead. The following codes works ok when the player is not jumping, but gets stuck in an object if the player is in the air when they do it. Left Key released event: if hsp = 0 exit; hspeed = -3; friction = 0.20; if obj_Player.hspeed = 0 { hspeed = 0; } Right key released event: if hsp = 0 exit; hspeed = +3; friction = 0.20; if obj_Player.hspeed = 0 { hspeed = 0; } and here's the horizontal collision code for interest: if (place_meeting(x+hsp,y,obj_bound)) { while(!place_meeting(x+sign(hsp),y,obj_bound)) { x += sign(hsp); } hsp = 0; } x += hsp; Any help would be much appreciated. Thanks.

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• How to get distance from point to line with distinction between side of line?

  - by tesselode

  I'm making a 2d racing game. I'm taking the nice standard approach of having a set of points defining the center of the track and detecting whether the car is off the track by detecting its distance from the nearest point. The nicest way I've found of doing this is using the formula: d = |Am + Bn + C| / sqrt(A^2 + B^2) Unfortunately, to have proper collision resolution, I need to know which side of the line the car is hitting, but I can't do that with this formula because it only returns positive numbers. So my question is: is there a formula that will give me positive or negative numbers based on which side of the line the point is on? Can I just get rid of the absolute value in the formula or do I need to do something else?

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• Detailed Modern Opengl Tutorial?

  - by Kogesho

  I am asking for a specific modern opengl tutorial. I need a tutorial that does not skip to explain any lines of code. It should also include different independent objects moving/rotating (most tutorials use only one object), as well as imported 3d objects and collision detection for them. It should also avoid stuff that won't be used. Arcysnthesis for example gives a new concept, and after teaching it, in the next tutorial, it explains how bad it is for performance and introduces another method. Do you know any?

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• Detecting pixels in a rotated Texture2D in XNA?

  - by PugWrath

  I know things similar to this have been posted, but I'm still trying to find a good solution... I'm drawing Texture2D objects on the ground in my game, and for Mouse-Over or targeting methods, I'm detecting whether or not the pixel in that Texture at the mouse position is Color.Transparent. This works perfectly when I do not rotate the texture, but I'd like to be able to rotate textures to add realism/variety. However, I either need to create a new Texture2D that is rotated at the correct angle so that I can detect its pixels, or I need to find some other method of detection... Any thoughts?

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• Vectors with Circles Physics -java

  - by Joe Hearty

  This is a problem I've been having, When making a set number of filled circles at random locations on a JPanel and applying a gravity (a negative change in the y), each of the circles collide. I want them to have collision detection and push in the opposite direction using vectors but i don't know how to apply that to my scenario could someone help? public void drawballs(Graphics g){ g.setColor (Color.white); //displays circles for(int i = 0; i<xlocationofcircles.length-1; i++){ g.fillOval( (int) xlocationofcircles[i], (int) (ylocationofcircles[i]) ,16 ,16 ); ylocationofcircles[i]+=.2; //gravity if(ylocationofcircles[i] > 550) //stops gravity at bottom of screen ylocationofcircles[i]-=.2; //Check distance between circles(i think..) float distance =(xlocationofcircles[i+1]-xlocationofcircles[i]) + (ylocationofcircles[i+1]-xlocationofcircles[i]) ; if( Math.sqrt(distance) <16)

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• 2D Game Development dynamics in c++ [on hold]

  - by novice

  I am new to developing computer graphic applications in c++ using OpenGl. I want to develop games but I really am facing problems when it comes to understanding concepts like trajectory, collisions, gravity and also the use of various physics engines that are available. when i search the internet I kind of get lost because they aren't for beginners like me. There is some hardcore mathematics, physics and coding involved. I need to pick the concepts that are mostly needed in game dev like trajectory, collision etc. Any good tutorials that can help me out in picking these concepts from the start.

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• Collisions and Lists

  - by user50635

  I've run into an issue that breaks my collisions. Here's my method: Gather Input Project Rectangle Check for intersection and ispassable Update The update method is built on object_position * seconds_passed * velocity * speed. Input changes velocity and is normalized if 1. This method works well with just one object comparison, however I pass a list or a for loop to the collision detector and velocity gets changed back to a non zero when the list hits an object that passes the test and the object can pass through. Any solutions would be much appreciated. Side note is there a more proper way to simulate movement?

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• Tile Collision Question

  - by Alu

  Hey guys, I'm working on tile collision. Currently, I just draw the tile map the normal way (two for loops) and there is no scrolling. Right now, to check if my player is over a tile, I use tileX = (int)person1v.X / 16; tileY = (int)person1v.Y / 16; However, I want to detect collision before I hit the tile so it could act as a wall. How do I detect collision before even making the move?

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• Why is there no plugin detection tecnique (like navigator.plugins) in PHP?

  - by ilhan

  Why is there no plugin detection tecnique (like navigator.plugins) in PHP? Why it is not implemented? It would be great! Any thoughts on how to do it? Edit: Like in $_SERVER['HTTP_USER_AGENT']

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• Beat detection and FFT

  - by Quincy

  So I am working on a platformer game which includes music with beat detection. I am currently using a simple if the energy that is stored in the history buffer is smaller then the current energy there is a beat. The problem with this is that ofcourse if you use songs like rock songs where you have a pretty steady amplitude this isn't going to work. So I looked further and found algorithms splitting the sound into multiple bands using FFT. I then found this : http://en.literateprograms.org/Cooley-Tukey_FFT_algorithm_(C) The only problem I'm having is that I am quite new to audio and I have no idea how to use that to split the signal up into multiple signals. So my question is : How do you use a FFT to split a signal into multiple bands ? Also for the guys interested, this is my algorithm in c# : // C = threshold, N = size of history buffer / 1024 public void PlaceBeatMarkers(float C, int N) { List<float> instantEnergyList = new List<float>(); short[] samples = soundData.Samples; float timePerSample = 1 / (float)soundData.SampleRate; int sampleIndex = 0; int nextSamples = 1024; // Calculate instant energy for every 1024 samples. while (sampleIndex + nextSamples < samples.Length) { float instantEnergy = 0; for (int i = 0; i < nextSamples; i++) { instantEnergy += Math.Abs((float)samples[sampleIndex + i]); } instantEnergy /= nextSamples; instantEnergyList.Add(instantEnergy); if(sampleIndex + nextSamples >= samples.Length) nextSamples = samples.Length - sampleIndex - 1; sampleIndex += nextSamples; } int index = N; int numInBuffer = index; float historyBuffer = 0; //Fill the history buffer with n * instant energy for (int i = 0; i < index; i++) { historyBuffer += instantEnergyList[i]; } // If instantEnergy / samples in buffer < instantEnergy for the next sample then add beatmarker. while (index + 1 < instantEnergyList.Count) { if(instantEnergyList[index + 1] > (historyBuffer / numInBuffer) * C) beatMarkers.Add((index + 1) * 1024 * timePerSample); historyBuffer -= instantEnergyList[index - numInBuffer]; historyBuffer += instantEnergyList[index + 1]; index++; } }

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• Points on lines where the two lines are the closest together

  - by James Bedford

  Hey guys, I'm trying to find the points on two lines where the two lines are the closest. I've implemented the following method (Points and Vectors are as you'd expect, and a Line consists of a Point on the line and a non-normalized direction Vector from that point): void CDClosestPointsOnTwoLines(Line line1, Line line2, Point* closestPoints) { closestPoints[0] = line1.pointOnLine; closestPoints[1] = line2.pointOnLine; Vector d1 = line1.direction; Vector d2 = line2.direction; float a = d1.dot(d1); float b = d1.dot(d2); float e = d2.dot(d2); float d = a*e - b*b; if (d != 0) // If the two lines are not parallel. { Vector r = Vector(line1.pointOnLine) - Vector(line2.pointOnLine); float c = d1.dot(r); float f = d2.dot(r); float s = (b*f - c*e) / d; float t = (a*f - b*c) / d; closestPoints[0] = line1.positionOnLine(s); closestPoints[1] = line2.positionOnLine(t); } else { printf("Lines were parallel.\n"); } } I'm using OpenGL to draw three lines that move around the world, the third of which should be the line that most closely connects the other two lines, the two end points of which are calculated using this function. The problem is that the first point of closestPoints after this function is called will lie on line1, but the second point won't lie on line2, let alone at the closest point on line2! I've checked over the function many times but I can't see where the mistake in my implementation is. I've checked my dot product function, scalar multiplication, subtraction, positionOnLine() etc. etc. So my assumption is that the problem is within this method implementation. If it helps to find the answer, this is function supposed to be an implementation of section 5.1.8 from 'Real-Time Collision Detection' by Christer Ericson. Many thanks for any help!

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• AABB Sweeping, algorithm to solve "stacking box" problem

  - by Ivo Wetzel

  I'm currently working on a simple AABB collision system and after some fiddling the sweeping of a single box vs. another and the calculation of the response velocity needed to push them apart works flawlessly. Now on to the new problem, imagine I'm having a stack of boxes which are falling towards a ground box which isn't moving: Each of these boxes has a vertical velocity for the "gravity" value, let's say this velocity is 5. Now, the result is that they all fall into each other: The reason is obvious, since all the boxes have a downward velocity of 5, this results in no collisions when calculating the relative velocity between the boxes during sweeping. Note: The red ground box here is static (always 0 velocity, can utilize spatial partitioning ), and all dynamic static collisions are resolved first, thus the fact that the boxes stop correctly at this ground box. So, this seems to be simply an issue with the order the boxes are sweept against each other. I imagine that sorting the boxes based on their x and y velocities and then sweeping these groups correctly against each other may resolve this issues. So, I'm looking for algorithms / examples on how to implement such a system. The code can be found here: https://github.com/BonsaiDen/aabb The two files which are of interest are [box/Dynamic.lua][3] and [box/Manager.lua][4]. The project is using Love2D in case you want to run it.

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